1. Field of the Invention
The present invention relates to a conversion curve setting system, which is utilized, for example, for an offset printing machine or the like, for setting a conversion curve, which defines openings of ink keys and the number of rotations of an ink fountain roller.
2. Description of the Related Art
In a printing machine, an amount of ink supplied on a surface of a sheet of paper or the like to be printed can be increased or decreased by the number of rotations of an ink fountain roller and openings of a plurality of ink keys, which are placed in the longitudinal direction of the ink fountain roller. Change in openings of the ink keys causes the gaps between the ink keys and the ink fountain roller to vary so as to adjust an amount of ink supplied. A preset operation for the number of rotations of the ink fountain roller and the openings of the ink keys is carried out prior to a start of printing operation, in order to obtain an optimum printing density. Substitution of the image area ratio, which is obtained by calculation of the image area for each of columns corresponding to the ink keys, into a predetermined conversion formula (i.e., a predetermined conversion curve) provides preset values for the openings of the ink keys and the number of rotations of the ink fountain roller.
With respect to a conversion curve setting system for setting such a conversion curve, there has conventionally been known an ink supply adjusting method in which there is conducted learning of the openings of the ink keys, by which a proper printing density can be obtained in an actual printing operation (Japanese Laid-Open Patent Application No. H7-117219). According to the above-mentioned ink supply adjusting method, a new target value is obtained by multiplying the opening of the ink key device, which is determined from the image area ratio, by correction factor stored by a correction factor storage device. In a correction factor modification device, there is conducted learning of the results of the adjusted opening of the ink key device by an operator, so as to modify the correction factor, thus permitting a proper printing in an actual operation. The above-described ink supply adjusting method makes it possible to cause the opening of the ink key device to approximate an optimum opening in case of carrying out a similar printing operation.
The ink supply adjusting method described above is suited for the similar printing operation. However, the adjusted openings of the ink keys may vary depending on ambient temperature, adjusting condition of components of the printing machine, a kind of ink to be used, a pattern to be printed and the like, even when the image area ratio is kept unchanged. As a result, it is difficult to modify the correction factor in the correction factor modification device in an appropriate manner, thus making it impossible to obtain the optimum openings of the ink keys.